The complex and damaging biochemical consequences of phospholipid peroxidation by free radicals and its inhibition by glutathione peroxidase will be studied using a suited model system for biological membranes. The effects of peroxidation of unsaturated phospholopid monolayers at the air-water interface, by a biologically relevant reaction such as the reduction of alloxan by ascorbic acid, will be monitered by measuring changes in surface pressure at constant area or changes in molecular area at constant surface pressure. The monolayer technique is ideally suited for such studies since the composition of the monolayer, the subphase below the interface, and the phase above the monolayer can be easily manipulated. This technique can provide important physiochemical information that is not readily obtainable by other methods. In general, monolayer expansion is an indication of ongoing peroxidation reaction. This allows for specific observation of changes in lipid molecule packing, orientation, conformation and fluidity upon peroxidation. The effect of adding other membrane components such as cholesterol and proteins can be also examined. The possible role of glutathione peroxidase in preventing membrane lipid peroxidation damage has been demonstrated in in vitro and in vitro studies. An extensive investigation of the surface properties of this enzyme at the air-water interface will be carried out in order to elucidate the interface conformation of glutathione peroxidase, its hydrophobicity and its surface activity. Penetration studies of the enzyme into phospholipid monolayers will investigate whether the enzyme which is water soluble can gain access to the unsaturated hydrocarbon chains region within the cell membrane. The ability of the enzyme to stabilize the membrane and prevent phospholipid peroxidation will be also assessed by subjecting phospholipid monolayers to peroxidative conditions in the presence of glutathione peroxidase. Such an investigation can provide information on whether the enzyme's role is limited to interaction with only hydrogen peroxide or whether its role is wider and it is capable of also interacting with lipid hydroperoxides.